feat(server): improve server startup error handling

Add structured error handling for server startup failures:
- Catch Bun.serve() errors and wrap in ServerStartError NamedError
- Pass through original Bun error messages (e.g., "Failed to start server. Is port 4096 in use?")
- Format errors in FormatError() to display user-friendly messages
- Prevents generic "unexpected error" messages for common server startup issues

packages/opencode/src/cli/error.ts

@@ -1,6 +1,7 @@
 import { Config } from "../config/config"
 import { MCP } from "../mcp"
 import { UI } from "./ui"
+import { Server } from "../server/server"
 
 export function FormatError(input: unknown) {
   if (MCP.Failed.isInstance(input))
@@ -13,4 +14,8 @@ export function FormatError(input: unknown) {
     ].join("\n")
 
   if (UI.CancelledError.isInstance(input)) return ""
+
+  if (Server.ServerStartError.isInstance(input)) {
+    return input.data.message
+  }
 }

packages/opencode/src/server/server.ts

@@ -766,12 +766,32 @@ export namespace Server {
   }
 
   export function listen(opts: { port: number; hostname: string }) {
-    const server = Bun.serve({
-      port: opts.port,
-      hostname: opts.hostname,
-      idleTimeout: 0,
-      fetch: app().fetch,
-    })
-    return server
+    try {
+      const server = Bun.serve({
+        port: opts.port,
+        hostname: opts.hostname,
+        idleTimeout: 0,
+        fetch: app().fetch,
+      })
+      return server
+    } catch (error) {
+      if (error instanceof Error) {
+        throw new ServerStartError({
+          port: opts.port,
+          hostname: opts.hostname,
+          message: error.message,
+        })
+      }
+      throw error
+    }
   }
+
+  export const ServerStartError = NamedError.create(
+    "ServerStartError",
+    z.object({
+      port: z.number(),
+      hostname: z.string(),
+      message: z.string(),
+    }),
+  )
 }